As one of electrostatic latent image developing methods, the developing method using, as a developer, composite particles produced by mixing and dispersing magnetic particles such as magnetite particles in a binder resin without using any carrier, i.e., a so-called “one-component magnetic toner” is extensively known and has been generally put into practice.
Recently, with the global spread and development of electrostatic copying machines and printing machines, it has been strongly required to improve properties of magnetic toners as a developer for these apparatuses, i.e., there is a strong demand for magnetic toners which are excellent in environmental stability so as to exhibit a stable developing performance even under any environmental conditions including high-temperature and high-humidity conditions and low-temperature and low-humidity conditions.
The magnetic toner mainly comprises magnetic particles and a binder resin, and the magnetic particles are uniformly dispersed in the toner particles. The electric field environmental conditions within the apparatuses when using such a toner are widely varied between respective makers of toners and developing devices for copying machines, printing machines, composite machines, etc. In the magnetic one-component developing system, black magnetic particles are mainly used as a pigment component for the developer. In this category, within the developing devices, it is most important that toner particles are allowed to exactly transfer from a toner thin layer formed on a developing sleeve charged in the range of from about hundred gausses to several thousands of gausses, to an electrostatic latent image portion as an electrostatically printed site finely formed on a photosensitive member, by controlling behavior of the toner particles. With such a measure, it is possible to obtain good toner images which are excellent in so-called thin-line reproducibility and gradation without any developing deficiencies such as fogging, insufficient image intensity, lack of images and uneven image intensity.
When the toner particles (or masses of the toner particles) are transferred from the developing sleeve to the surface of the photosensitive member, two forces, i.e., a magnetic force (acting between a pigment in the developer particles and a magnetic pole of the sleeve) and an electrostatic attraction force (acting between a surface of the respective developer particles charged by friction electrification and an electrostatic latent image formed on the surface of a photosensitive drum) are exerted on the toner particles.
More specifically, in order to well control behavior of the toner particles, in the process from frictional electrification of the toner particles through desorption of the toner particles from a surface of the sleeve up to adhesion of the toner particles to the surface of the electrostatic latent image portion on the photosensitive member, the procedure of controlling the behavior of the toner particles must be carried out while keeping a good balance between an electrostatic amount (charge amount) and a magnetic force of the toner particles.
First, as the premise for well controlling the behavior of the toner particles, it is desirable that a surface potential (charge amount) and the magnetic force of the individual toner particles are kept uniform.
In view of the uniform magnetic force, it is important that the magnetic material is incorporated in a uniform amount in the individual toner particles. Simultaneously, it is desired that the magnetic material is uniformly dispersed in the toner particles and is hardly desorbed from the surface of the respective toner particles.
On the other hand, in view of the uniform charge amount, it is desired that the toner particles are instantaneously uniformly charged by frictional electrification up to a desirable surface potential and maintain the charge amount.
The magnetic particles are exposed to the surface of the respective toner particles, and act as a so-called leak site through which the surface potential produced by the frictional electrification is escaped into atmospheric air, because the magnetic particles have a property of a semiconductor which is apt to flow an electric current therethrough as compared to other constituents of the toner.
Therefore, the most important task resides in that the toner is capable of maintaining a uniform charge amount on a surface thereof even under high-temperature and high-humidity conditions, and there is a strong demand for a magnetic material having a high electrical resistance value so as not to exhibit such a condition in which the toner acts as a leak site owing to exposure of the magnetic material to the surface of the toner and thereby fails to maintain a uniform charge amount.
As described above, when the toner particles are transferred from the developing sleeve to the surface of the photosensitive member, the two forces, i.e., the magnetic force and electrostatic attraction force, are exerted on the toner particles. First, the developer particles must be desorbed in the form of masses of ultrafine particles from the surface of the sleeve where a magnetic field predominates, into a space in which the above two forces act.
At this time, as a force of propelling the desorption of the toner particles, a bias voltage is applied thereto. The bias voltage is usually in the form of a superimposed voltage of a direct current bias and an alternating current bias. In general, the bias voltage is lower than a voltage of a portion of the surface of the photosensitive drum other than the latent image-forming portion thereof.
In addition, the bias voltage is also used to control the behavior of the toner particles in the space when masses of the toner particles are desorbed from the sleeve and transferred to the electrostatic latent image-forming portion on the surface of the photosensitive member. The frequency of the bias voltage is set variously depending upon design of the machine used, and is usually about several tens of Hz to about 20 kHz.
When the frequency of the bias applied is excessively high, the toner particles hardly follow the variation of the bias. Therefor, the frequency of the bias applied is designed so as to have a good balance with the movement of the toner particles.
In order to allow the toner particles to maintain the uniformly charged condition without leakage of the charge on the surface of the toner in the electric field, it is desired that the magnetic material exposed to the surface of the respective toner particles has a high electrical resistance value. In this regard, the consideration of a direct current resistance value only will be insufficient, and the electrical resistance value in an alternating current bias space, i.e., impedance, should also be taken into consideration.
Hitherto, there have been made various studies on oxides having a high impedance. For example, the oxides include titanium oxide and titanate compounds as typical dielectric substances. In addition, as to the magnetic materials in the form of oxides having a high impedance, studies have also be made in the fields of radio wave absorbers, high-frequency magnetic cores and soft ferrite ceramics.
Magnetite is basically a semiconductor material. It is considered that the electric conductivity of magnetite is produced based on hopping conductivity between Fe2+ and Fe3+.
For this reason, in order to increase the resistance value of magnetite, the present inventors have past attempted that the surface of magnetite particles is treated with a dielectric substance component; the Fe2+ component therein is substituted with the other elements; the surface of magnetite particles is oxidized to convert Fe2+ into Fe3+; or the structure within magnetite particles or near the surface of the magnetite particles becomes close to that of Zn ferrite, etc.
However, when the magnetite particles are surface-treated with the fine particles, the resulting particles tend to have a large surface area, so that a moisture absorption thereof tends to be undesirably increased. Further, it has been found that when Fe2+ is reduced or is substituted with the other elements, it is not possible to ensure a high blackness degree as an inherent feature of magnetite. In addition, the method of incorporating the heavy metal elements into the magnetite is undesirable from the environmental viewpoints.
As a result of the present inventors' earnest study, there has been developed a black magnetic material with a high blackness which comprises safe elements and has a high impedance.